The present invention relates to an improved melt polymerization process for making polyetherimides. Melt polymerization methods for making polyetherimides (e.g., see U.S. Pat. No. 3,803,085, Takekoshi and Kochanowski) generally comprise effecting the removal of water of reaction at temperatures up to 400.degree. C. from a melted mixture containing, as essential ingredients, an organic diamine of the formula, EQU H.sub.2 NRNH.sub.2
and an aromatic bis(ether anhydride) of the formula, ##STR1## where R is a divalent organic radical selected from the class consisting of (a) aromatic hydrocarbon radicals having from 6 to about 20 carbon atoms and halogenated derivatives thereof, (b) alkylene radicals and cycloalkylene radicals having from 2 to about 20 carbon atoms, and (c) C.sub.(2-8) alkylene terminated polydioganosiloxanes, and divalent radicals induced by the formula ##STR2## where Q is a member selected from the class consisting of ##STR3## where x is an integer from 1 to about 5 inclusive. The --O--A&lt; group is selected from ##STR4## where R' is hydrogen, lower alkyl or lower alkoxy, preferably the polyetherimide includes the latter --O--A&lt; group, wherein R' is hydrogen. Z is a member of the class consisting of ##STR5## and (2) divalent organic radicals of the general formula ##STR6## where Y is a member selected from the class consisting of divalent radicals of the formulas, ##STR7## where q is 0 or 1, and y is an integer from 1 to about 5. In preferred polyetherimides --O--A&lt; group is ##STR8## Thus, the resulting polyetherimide has the formula ##STR9## wherein Z and R are as defined above, the divalent bonds of the --O--Z--O-- radical are in the 3,3'; 3,4'; 4,3' or the 4,4' positions, and "a" is an integer greater than 1, e.g., from 10 to 10,000 or more. In particularly preferred polyetherimides, Z is a group of the formula ##STR10## and R is selected from ##STR11## The polyetherimides where R is m-phenylene are most preferred.
In U.S. Pat. Nos. 3,833,546 and 4,011,198, Takekoshi et al. disclose that the above melt polymerization can be performed in a continuous manner by an extrusion melt polymerization process wherein a reaction mixture containing as essential ingredients as organic diamine and a bis(ether anhydride) are continuously fed through an inlet opening into the barrel of a screw extruder maintained at melt polymerization temperatures, and water of reaction is continuously vented from the extruder. The above-cited patents are incorporated herein by reference.
In U.S. Pat. No. 4,073,733, a process is disclosed wherein a mixture of an organic diamine and an aromatic bis(ether anhydride) is continuously fed through an inlet opening into a screw extruder having a second opening downstream from the inlet opening. The mixture is passed through a first extruder zone maintained at a low temperature to a zone where the mixture is melted and water of reaction is continuously removed through the second opening. A melt seal may be employed between the second opening and a third opening through which water of reaction may be removed under vacuum.
During conventional melt polymerization processes, the polymerization mixture passes through a so-called "cement stage." At this stage of polymerization, the melt becomes extremely viscous and difficult to process. Extruder torsion forces sometimes become so high as to cause equipment failure during this stage. Accordingly, there is a need for a melt polymerization process for producing polyetherimides in which the high viscosities associated with the "cement stage" are reduced or eliminated.